In optical communication fields or optical measurement fields, a variety of optical modulators such as intensity modulators including Mach-Zehnder type optical waveguides are being used. The intensity changes of light being output from Mach-Zehnder type optical waveguides show sinusoidal characteristics with respect to voltages being applied to modulation electrode. In order to obtain appropriate intensities of output light in accordance with the use of optical modulators, it becomes necessary to set modulation signal which is applied to modulation electrode to appropriate operating bias point.
Therefore, in the related art, some of signal light which is output from optical modulator or radiated light which is radiated from combining part in Mach-Zehnder type optical waveguides is detected as monitoring light using light receiving element such as photo detector, and the intensity state of output light from optical modulator is monitored. In addition, the operating bias point of modulation signal which is applied to modulation electrode is adjusted (bias-controlled) on the basis of detection values (monitor output) of light receiving element.
Patent Literature No. 1 discloses an optical modulator in which radiated light is monitored using a light receiving element 5 disposed outside a substrate 1. Specifically, in the substrate 1 having an electro-optic effect, an optical waveguide 2 including a Mach-Zehnder type optical waveguide (21 to 24) is formed. Modulation electrode for modulating light waves that propagate through the optical waveguides is provided along two branching waveguides which configure the Mach-Zehnder type optical waveguide, which are not illustrated in the drawing. The output waveguide 24 has an optical fiber 4 connected thereto and is constituted to output outgoing light to the outside.
Two radiated lights (R1 and R2) being radiated from a combining part 23 in the Mach-Zehnder type optical waveguide pass through the inside of a capillary 3 for reinforcement to connect the optical fiber 4 to an end portion of the substrate 1 and are introduced into the light receiving element 5. Particularly, as illustrated in FIG. 1, By configuring for two radiated lights to be detected by a single light receiving element, it is possible to monitor two radiated lights in a combined state, and to compensate for phase difference between monitoring light and output light S.
In order to improve the accuracy of compensating phase difference between monitoring light and output light, it is essential to optimally set light intensity ratio at which two radiated lights are received. Therefore, any light intensity ratio-adjusting means is provided between the capillary 3 and the light receiving element 5 in FIG. 1.
Patent Literature No. 2 discloses a configuration in which the light receiving element 5 is disposed on the substrate 1 on which an optical modulator is configured as illustrated in FIG. 2 and FIG. 3. Specifically, in the substrate 1, the optical waveguide 2 including a Mach-Zehnder type optical waveguide and a modulation electrode (not illustrated) for modulating light waves that propagate through the optical waveguide are formed. The light receiving element 5 is disposed to bridge over the output waveguide 24 which configures the Mach-Zehnder type optical waveguide.
In FIG. 2, the light receiving element 5 is configured to receive two radiated lights being radiated from the combining part in the Mach-Zehnder type optical waveguide together. The radiated lights propagate through the inside of the substrate 1; however, in order to precisely control locations through which radiated light propagates, it is possible to provide waveguides for radiated light (25 and 26) which guide radiated light. The light receiving element 5 is disposed so as to span the two waveguides for radiated light (25 and 26).
FIG. 3 is a sectional view in a direction of a dash-dot line X-X′ in FIG. 2. High-refractive-index films (40 and 41) are disposed in contact with or close to the waveguides for radiated light (25 and 26), whereby some (R1 and R2) of radiated lights are absorbed toward the light receiving element 5 and are incident on an optical detecting area 50.
In the configuration of FIG. 2 and FIG. 3, two radiated lights can be received at the same time, and, as described in an example of the related art of FIG. 1, phase difference between monitoring light for which radiated light is used and output light which comes from the output waveguide is compensated by adjusting the light-receiving intensities of the two radiated lights and by receiving two radiated lights simultaneously so that it becomes possible to obtain favorable monitoring characteristics.
As disclosed in Patent Literature No. 1, in a case in which the light receiving element 5 is disposed outside the substrate 1, in a case in which plural of Mach-Zehnder type optical waveguides are integrated on the same substrate or a case in which a Mach-Zehnder type optical waveguide is formed at a location away from an end surface of a substrate and another optical waveguide is present between the Mach-Zehnder type optical waveguide and a capillary, it becomes extremely difficult to guide only intended light waves to the light receiving element.
In contrast, as disclosed in Patent Literature No. 2, the configuration in which the light receiving element is disposed on the surface of the substrate 1 has an advantage that it is possible to precisely receive intended light waves. However, in order to receive two radiated lights at the same time, it becomes necessary to form a light-receiving area having a size that is approximately identical to or larger than the gap W between the two waveguides for radiated light (25 and 26) illustrated in FIG. 3. An increase in the area of optical detecting area causes a problem of frequency bandwidth of light receiving elements being decreased in inverse proportion to the size thereof.